Parabolas - lindagreen.web.unc.edu · • Define a parabola as the set of points that are...

PARABOLAS

Parabolas

After completing this section, students should be able to:

• Define a parabola as the set of points that are equidistant from a point (its focus)and a line (its directrix).

• Given the focus and directrix of a parabola, or the focus and vertex, or the vertexand directrix, write down its equation in the form (x � h)2 = 4p(y � k) or (y � k)2 =4p(x � h).

• Graph a parabola given in the form (x � h)2 = 4p(y � k) or (y � k)2 = 4p(x � h) andlocate its focus, directrix, and axis of symmetry.

• Given an equation of a parabola in a general form like 4x�20x�8y+57 = 0, rewriteit in a standard form (x � h)2 = 4p(y � k) or (y � k)2 = 4p(x � h).

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PARABOLAS

Example. Find the equation of the points equidistant from the point (0, p) and thehorizontal line y = �p.

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PARABOLAS

Example. Find the equation of the points equidistant from the point (p, 0) and thevertical line x = �p.

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PARABOLAS

Example. Find the equation of a parabola with vertex at (h, k), assuming the parabolaopens up or down.

Example. Find the equation of a parabola with vertex at (h, k), assuming the parabolaleft or right.

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PARABOLAS

Example. Find the equation of a parabola with vertex at (2, 4) and focus at (�1, 4).Graph the parabola.

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PARABOLAS

Review. Which of the following represents the equation of a parabola?

A. y = ax2 + bx + c

B. y = a(x � h)2 + k

C. 4p(y � k) = (x � h)2

D. (x � h)2 + (y � k)2 = r2

Review. Which of the following represents the equation of a parabola that opens tothe left?

A. 8(y � 5) = (x + 2)2

B. y + 1 = �4(x � 3)2

C. 12(x + 6) = (y � 5)2

D. x � 3 = �12

(y + 2)2

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PARABOLAS

Fill out the chart based on the figures.

Opens up or down Opens left or right

Equation

Vertex

Focus

Directrix

Axis of symmetry

Graph (p > 0)

Graph (p < 0)

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PARABOLAS

Example. Find the equation of a parabola with vertex at (1, 6) and directrix at x = 9.

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PARABOLAS

Extra Example. Find the equation of a parabola with focus at (�3, 2) and directrix aty = 8.

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PARABOLAS

Example. Find the focus, directrix, and vertex of this parabola. Then graph theparabola.

x2 � 6x � 2y + 1 = 0

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PARABOLAS

Extra Example. Find the focus, directrix, and vertex of this parabola. Then graph theparabola.

�y2 + 8y + 5x � 31 = 0

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PARABOLAS

Example. Find the equation of the parabola.

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PARABOLAS

Extra Example. Find the equation of the parabola.

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PARABOLAS

Example. A satellite dish has the shape of a paraboloid that is 10 feet across at theopening and is 3 feet deep. At what distance from the center of the dish should thereceiver be placed to receive the greatest intensity of sound waves?

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PARABOLAS

Example. The Golden Gate Bridge, a suspension bridge, spans the entrance to SanFrancisco Bay. Its towers rise 526 feet above the road and are 4200 feet apart. Thebridge is suspended from two cables. The cables are parabolic in shape and touch theroad surface at the center of the bridge. Using the location where the cables touch theroad surface as (0,0), find the height of the cable 200 feet from the center of the bridge.

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PARABOLAS

Extra Example. Which of the following equations are equivalent to y =32

x2+15x+692

?

A. y =32

(x + 5)2 � 3

B. (x + 5)2 =23

(y + 3)

C. 3x2 + 30x � 2y + 69 = 0

D. None of these.

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ELLIPSES

Ellipses

After completing this section, students should be able to:

• Define an ellipse in terms of distances to the two foci.

• Given an equation of an ellipse in a form like(x + 4)2

16+

(y � 1)2

4= 1, find its center,

vertices, foci, and major and minor axes.

• Given an equation in a form like 4x2 + 9y2 + 8x � 36y = 6, complete the square to

put it in the form(x � h)2

a2 +(y � k)

b2 = 1

• Given an equation in the form the form(x � h)2

a2 +(y � k)

b2 = 1, draw its graph.

• Write down the equation for an ellipse from its graph.

• Find the equation of an ellipse from the coordinates of its foci and one of its vertices,or similar information.

• Solve applications problems involving ellipses.

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ELLIPSES

Recall:. A circle can be defined as the set of points ...

Note. An ellipse can be defined as the set of points ...

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ELLIPSES

Features of an ellipse

Example. Find the equation of an ellipse with foci at (�c, 0) and (c, 0) and vertices at(�a, 0) and (a, 0).

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ELLIPSES

Summary: For a > b, the equationx2

a2 +y2

b2 = 1 represents ...

For a > b, the equationx2

b2 +y2

a2 = 1 represents ...

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ELLIPSES

What about an ellipse centered at (h, k)?

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ELLIPSES

Example. Write the equation of the ellipse drawn. Find its foci.

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ELLIPSES

Review. Which of the following represents the equation for an ellipse?

A. 9x2 + 4y2 = 36

B.x2

25� y2

100= 1

C. 4x2 + 6y2 + 8x + 12y = 15

D.(x � 3)2

4+

(y + 1)2

9= 1

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ELLIPSES

Example. Graph the ellipsex2

4+

y2

9= 1.

Example. Graph the ellipse(x � 3)2

4+

(y + 1)2

9= 1.

What are the center, vertices, and foci?

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ELLIPSES

Example. Graph the ellipse 4x2 + 9y2 � 16x + 18y � 11 = 0.

What are the center, vertices, and foci?

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ELLIPSES

Example. Find an equation for the ellipse with foci at (�4, 2) and (�4, 8) and vertex at(�4, 10).

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ELLIPSES

Extra Example. Find the equation of an ellipse with a minor axis of length 6 and acenter at (�3, 3) and a vertex at (1, 3)

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ELLIPSES

Extra Example. Write the equation of the ellipse drawn.

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ELLIPSES

Extra Example. A one-way road passes under an overpass in the form of half of anellipse, 15 feet high at the center and 20 feet wide. Assuming a truck is 12 feet wide,what is the height of the tallest truck that can pass under the overpass?

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ELLIPSES

Extra Example. A satellite is placed in elliptical earth orbit with a minimum distanceof 160 km and a maximum distance of 16000 km above the earth. Find the equation ofthe orbit of the satellite, assuming that the center of the earth is at one focal point andthat the radius of the earth is 6380 km.

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HYPERBOLAS

Hyperbolas

After completing this section, students should be able to:

• Define a hyperbola in terms of the distances to the two foci.

• Given an equation of a hyperbola in a form like(x + 4)2

16� (y � 1)2

4= 1, find its

vertices and center.

• Given an equation in a form like 4x2 � 9y2 + 8x � 36y = 6, complete the square to

put it in the form(x � h)2

a2 � (y � k)b2 = 1

• Given an equation in the form the form(x � h)2

a2 � (y � k)b2 = 1, draw its graph.

• Write down the equation for a hyperbola from its graph.

• Find the equation of a hyperbola from the coordinates of its foci and one of itsvertices, or similar information.

• Solve applications problems involving hyperbolas.

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HYPERBOLAS

Definition. A hyperbola is the set of points (x, y) such that ...

Features of a hyperbola

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HYPERBOLAS

Example. Find the equation of an hyperbola with foci at (�c, 0) and (c, 0) and verticesat (�a, 0) and (a, 0).

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HYPERBOLAS

Summary: The equationx2

a2 �y2

b2 = 1 represents ...

The equationy2

a2 �x2

b2 = 1 represents ...

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HYPERBOLAS

What about an hyperbola centered at (h, k)?

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HYPERBOLAS

Example. Graph the hyperbola(x � 6)2

4� (y + 3)2

25= 1.

Find its center, vertices, foci, and asymptotes.

END OF VIDEO

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HYPERBOLAS

Review. For the equation(x � h)2

a2 � (y � k)2

b2 = 1.

1 Which way is the hyperbola oriented?

OR

2 The center is at ...

3 a represents ...

4 c represents ...

5 c is related to a and b by the equation ...

6 The slope of the asymptotes is:

7 For hyperbolas, a is (circle one) the larger number / the number that goes with thepositive term. (What about for ellipses?)

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HYPERBOLAS

Which answers change for the equation(y � k)2

a2 � (x � h)2

b2 = 1 ?

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HYPERBOLAS

Example. Find the center, vertices, foci, and asymptotes for(x � 1)2

9� (y + 3)2

4= 1.

Graph the hyperbola.

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HYPERBOLAS

Example. Find the center, vertices, foci, and asymptotes for the hyperbolay2 � 6y � 4x2 � 32x � 59 = 0. Graph the hyperbola.

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HYPERBOLAS

Example. Write the equation of a hyperbola with foci at (0,�3) and (�4,�3) and avertex at (�3,�3).

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HYPERBOLAS

Extra Example. Find an equation of the hyperbola having foci at (3, 8) and (3, 12) andvertices at (3, 9) and (11, 3)

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HYPERBOLAS

Extra Example. Shearon Harris Nuclear Plant in North Carolina has a hyperboloidcooling tower 526 feet tall. Assuming that the Harris cooling tower is 400 feet wideat its base and is 250 feet wide at its narrowest point 400 o↵ the ground. What is theequation of the hyperbola that models the shape of this tower?

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HYPERBOLAS

Extra Example. What is the equation of the hyperbola that models the light pattern onthe wall?

.

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HYPERBOLAS

Question. What shapes can you get by slicing a double cone with a plane?

For example, if you cut it with a horizontal plane, you will get a circle.

If you tilt the plane, or make it vertical, what other shapes can you get?

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HYPERBOLAS

Example. Identify each conic section.

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HYPERBOLAS

Example. Identify each conic section.

A. x2 � 8y2 � x � 2y = 0

B. 2x2 � y + 8x = 0

C. x2 + 2y2 + 4x � 8y + 2 = 0

D. 4x2 + 4y2 � x + 18y � 17 = 0

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